net-sysfs: warn if new queue objects are being created during device unregistration

Calling netdev_queue_update_kobjects is allowed during device
unregistration since commit 5c56580b ("net: Adjust TX queue kobjects
if number of queues changes during unregister"). But this is solely to
allow queue unregistrations. Any path attempting to add new queues after
a device started its unregistration should be fixed.

This patch adds a warning to detect such illegal use.
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net-sysfs.c

@@ -1694,6 +1694,13 @@ netdev_queue_update_kobjects(struct net_device *dev, int old_num, int new_num)
 	int i;
 	int error = 0;
 
+	/* Tx queue kobjects are allowed to be updated when a device is being
+	 * unregistered, but solely to remove queues from qdiscs. Any path
+	 * adding queues should be fixed.
+	 */
+	WARN(dev->reg_state == NETREG_UNREGISTERING && new_num > old_num,
+	     "New queues can't be registered after device unregistration.");
+
 	for (i = old_num; i < new_num; i++) {
 		error = netdev_queue_add_kobject(dev, i);
 		if (error) {